Secondary emission circuit for amplitude discrimination



Sept. 5, 1950 c. H. HOEPPNER 2,520,932

SECONDARY EMISSION CIRCUIT FOR AMPLITUDE DISCRIMINATION Filed April 26,1945 2 Sheets-Sheet 1 I .I. E; L

I FRSIIJEZNCY DISCR'MINATION RECEIVER STAGE STAGE PULSE RECEIVING SYSTEMgwuam/kw CONRAD H. HOE PPNER v 2% R M W Sept. 5, 1950 c. H. HOEPPNERSECONDARY EMISSION CIRCUIT FOR AMPLITUDE DISCRIMINATION 2 Sheets-Sheet 2Filed April 26, 1945 ouuzscam TUBE 2O GUT OF F 3mm- CONRAD H. HOEPPN ERPatented Sept. 5, 1950 SECONDARY EMISSION CIRCUIT FOR AMPLITUDEDISCRIMINATION Conrad H. Hoeppner, Washington, D. 0.

Application April 26, 1945, Serial No. 590,444

(Granted under the act of March a, 1883, as amended April 30, 1928; 3700. G. 157) Claims.

This invention relates in general to electronic circuits havingdiscriminatory response characteristics and in particular to anelectronic circuit for amplitude discrimination.

In radio, radar, television and other electronic fields, it frequentlyoccurs that a number of different potential variations may exist at theinput to a component electronic circuit either fortuitously or byintention. If all of such potential variations are not to be impressedupon the component circuit, it is necessary to provide an interveningcircuit with the ability to discriminate between those variationsintended for ultimate application to the component circuit and thosevariations the effect of which would be undesirable. Somecharacteristics of the potential variations must be selected as a basisfor pulse discrimination and among such characteristics are timeduration, rate of voltage change, and amplitude.

It is an object of this invention to provide a circuit which isresponsive only to potential variations or electrical impulses of acertain amplitude and polarity and un-responsive to potential variationsor electrical impulses of all other amplitudes or polarity.

It is another object of this invention to provide a circuit which can beemployed between a source of potential variations or electricalimpulsesand the receiver thereof as an intervening circuit which shields fromsuch receiver all variations or pulses except those having a certaindefinite preselected amplitude and polarity.

It is another object of this invention to provide a discriminationcircuit the discriminatory action of which is based upon certaindefinite characteristics of the applied input signal.

Other objects and features of this invention will become apparent upon acareful consideration of the following detailed description when takentogether with the accompanying drawings in which:

Fig. 1 is a simple block diagram of a system in whichone embodiment ofthis invention is utilized.

Fig. 2 is a circuit diagram of the embodiment of this invention shown inFig. 1.

Figs. 3 and 4 are wave forms employed in explaining the operation of thecircuit shown in Fig. 2.

Reference is now had in particular to Fig. 1 wherein there is shown oneembodiment of this invention in which a discrimination circuit isemployed to reject undesired video signals in a. pulse receiving system.Pulses or bursts of high frequency energy received by antenna I,amplified and detected by high frequency stage 2 are impressed, in theform of .the envelope of the high frequency pulses to input 3 of thediscrimination stage 4. Since the pulses of high frequency energyreaching antenna I may comprise not only a desired signal but alsoman-made and fortuitous interfering signals of a frequency which stage 2will not reject, and since high frequency stage 2 may itself be a sourceof interfering signal, it is the function of the discriminination stage4 to shield from receiver 5 all pulses not having the ampltiude (andpolarity) characteristics of the desired signal.

Reference is now had to Fig. 2 wherein there is shown one form of anampltiude discrimination circuit constructed according to the teachingsof this invention and which serves as discrimination stage 4 of thepulse reception equipment illustrated in Fig. l. r

In Fig. 2, the positive potential 8 to which screen grid 1 of fourelectrode vacuum tube 6 is connected is more positive than potential 9to which anode I0 is connected through resistance [3' thereby creatinga, negative potential gradient from screen 1 to anode l0. Control gridI2 is normally biased sufliciently by connection through resistance I4to negative potential l5 to hold tube 6 in a non-conducting condition.The coupling network comprising capacitor l6 and resistance I4 permitsthe application of signals from high frequency stage '2 which, if ofsufficient amplitude and of positive polarity, raise the potential ofgrid l2 with respect to cathode l l high enough to permit the fiow ofelectrons toward screen I. Obviously, any signal of amplitude less than.that just described, or of negative polarity, will fail to disturb thequiescent condition of tube 6. The coupling network comprising capacitorl6 and resistance M has a time constant which is long with respect tothe time dura tion of any signal impressed upon input 3so that capacitorIt assumes very little charge during any signal. Thus vacuum tube 6 andits associated components function to discriminate against electricalimpulses from high frequency stage 2 of negative polarity and those ofpositive polarity of less than a pre-determined amplitude.

Positive pulses applied to input 3 and having an amplitude in the rangebetween that which will initiate space current flow in tube 6 and thatwhich will undertake to raise grid l2 above cathode II in potentialwill, however, disturb the quiescent condition of tube 6. The potentialdrop caused by flow of grid current through lify any attempt to raisegrid I2 above cathode II and thereby places an upper limit on thedisturbance of the quiescent condition of tube 6. The behavior of thepotential variation appearing at anode II! between the lower and upperlimits just described enables'the circuit of Fig. 2 to discriminateagainst positive pulses impressed upon input 3 of an amplitude either .4In a discrimination circuit constructed according to the teachings ofthis invention, as shown in one form in Fig. 2, and employing one ofseveral common receiving type multi-electrode 5 vacuum tubes (such asSAC? pentode with suppressor direct connectedto the screen grid to forma four electrode tube) as tube 5, thesecondary current will exceed theprimary current and thus give a ratio less than unity over a rangegreater or less than'a certain preselected ampli It from a virtuallynon-conducting condition of tube tude in a manner which will be in thefollowing paragra h When tube 5 is in a conducting condition, the

electrons which leave the space charge surrounding cathode II traveltoward screen .1 and anode II] under the accelerating influence of thepositive potential gradient between grid I2 and screen I. The density ofthe electron stream is controlled almost entirely byvariations in the. V

below a, certain predetermlned amplitude conpotential of grid I2 sincescreen grid I is held at a fixed potential by direct connection tosource 8 and acts'in the normal mannerto shieldQthe cathode space"charge from potential'variations whicnmay appear at anode II] by virtueof current flow through resistance I3. .The electron stream is dividedinto several groups after it passes grid I2. The first of thesegroupscollides directly with the metallic ele- "ments of screen grid Iand comprises a part of 'the jscreen grid current. This collision is ata "velocitywhich is sufficient to cause electrons to be; emitted fromthe surface of screen grid .1, the; number of which is determinedessentially 'by' 'the velocity of bombardment and the work functioningenergy of the screen grid surface.

All such bombardment emitted electrons, which' "constitute secondaryemission, are returned to screen grid 1 since it is the mostpositiveelec The second of these groups,

trode intube 6. i passes through the interstices of screen grid. 1

with insufiicient velocity to overcome the negative potential gradientexisting between screen grid 1 and anode I I) and arethus turned back toscreen grid I and constitute'ano ther part of the screen grid current.The third of these groupspasses through the interstices of screen grid Iwithsufficient velocity to continue on to anode I0 and compriseconventional plate current "flowing in such a direction throughresistance I3 as to reduce the potential of anode Ill if allowed to actindependently. This conventional plate current can be termed primarycurrentto dis tin guish it from the current described below which flowsas the result of secondary emission of electrons from anode I I3. Themajority of the electrons which reach anode In from the space tential atanode I I] and they may therefore be termed secondary current. It willbe apparent that the potential of anode ID will be determined by theratio of primary to secondary current. The potential of anode III oftubefi will bejnegative with respect to the quiescent or cutofi po.-tential if theratio is greater than unity and will be positive withrespect to quiescent potential if the ratio is less than unity..

6 up to a density of space current electron flow -wh-ich so alters thepotential gradient between screen grid I and anode II] that enough ofthe secondary electrons emitted from anode II} are l -5 returned toanode IE) to permit the proportion of primary current to secondarycurrent to assume a value of unity or greater. This means that positivepulses above a certain predetermined amplitude controllable by potentialI5 and by said potentials 8 and 9; primary exceeds secondary currentandthe potential of anode It moves in a negative direction from itsquiescent 7 value. i V

In wave form I8 of Fig. 3, net plate current 130 (1 has been plotted asthe vertical coordinate contains both'la negative region between pointsd and b and a positive region between points 2; and d. ,At gridpotential a and below, space current fio w isrnegligflole. In the gridpotential region'froim' ajto 'c the primary current is less thansecondary current. The variation in anode II) potentialiE withvariations in grid I2 potential (Eg) is shown in wave form l9. A 'fiAC'l 'operat- 'ing with '-'[-300'volts on its screen grid I and +100,voltsfion its plate I0 gave a transconductance characteristic similarto that shown by wave form "I8." In this connection it may be well tostate that variations in the voltage gradient between the screen grid Iand the anode I0 gave both a different slope to the negativetransconductance 5,0 portion a. to b of the tnansconductancecharacteristic and also a different curvatureto'the inflec tion point b.The greater this voltage gradient the steeper the slope of the negativetranscondu-ctance portions .and the sharper thecurvature of theinflection point b with optimumperformance attainable with the above.voltage parameters. a r r In Fig. 2 vacuum tube'ZII and associatedcircuit components compriseone form of .abonventional amplifyingfcircuit connected sothat the potential variations appearing at anode IUof tube 6 are applied as signals to grid 2| via the {coupling circuitcomprising capacitor ZZand resistance 23. Potential -2 4, to which grid'2| is connected v I through resistance 23, is such that tube 20 isnoncurrent, their flow is such as to increasethe poconducting-in thequiescent condition and requires a positive signal on grid 2! beforeplate current can flow. The flow or-platescurrent through resistance 25in response to a positive 7 signal applied to grid 2| causes a negativesignal to appear at plate 26 and thus at output 21 of the discriminatingcircuit of- Fig. 2. Negative signals from anode I0 applied to-grid 2Iwill only drive tube 20 further below cutoff and will be p I impotentinsoiar as anyoutput at 21 is concerned;

In operation, potential I 5, the source of grid bias for tube 6, is sofixed that a positive pulse having the amplitude of a desired'signalwill, when applied at input 3, cause a net plate current flow which issuch as to cause a rise in potential at anode ll]. This rise, applied togrid 2| of tube 23, causes plate current flow and thus a negative signaloutput at 21. An interference positive pulse at input 3 of amplitudeless than a certain value will not distrub the quiescent condition oftube 6. An interference signal of amplitude greater than a certain valuecauses a net plate current flow which is such as to bring about adecrease below quiescent potentialat anode I0.

This negative going variation applied to grid 2| of tube 20 has noefiect as hereinbefore explained. It will be obvious that potential 24,the source of grid bias for tube 20, can be fixed at a point such thatonly the most positive swings of anode Ii! will cause a signal to appearat output 21. Operation in this fashion is illustrated in Fig. 4, inwhich wave form 28 is representative of a series of electrical impulsesapplied to input 3 of Fig. 2. In this series, pulse g represents thedesired signal, while pulses d, e, f, and h represent interferencesignals. The behavior of tube 6 is response to these pulses, asexpressed by the potential at anode H3 is shown by wave form 29. On thiswave form has been shown the level to which anode It must rise beforeplacing tube 20 in a conducting condition. The behavior of tube 20 inresponse to the signals from anode I0, as expressed by the output at 21,is shown by wave form 30.

With reference to wave form 28, pulse (1 is of insufficient amplitude tocause conduction by tube 6 and does not, therefore, affect the output ofthe discrimination circuit. Pulse 6 is of an amplitude so great that thepotential of grid 12 of tube 6 is driven into the positivetransconductance region and to a point between and d of Fig. 3 in whichnet plate current flow causes a negative signal to appear at anode Ill.As previously explained, this negative potential change at anode [0 onlybiases tube 20 further into the non-conducting state and thus pulse edoes not affect output 2'! of the discriminating circuit. Pulse 1 is ofan amplitude such that the potential of grid l2 of tube 5 is driven intothe negative transconductance region a to b of Fig. 3, such as potential9'. Potential 7' at grid I2 will cause a net plate current flow suchthat anode I0 is positive with respect to quiescent potential but thepositive swing of anode I0 is less than that which would have occurredhad grid [2 been driven to the inflection point D of thetransconductance characteristic. Since potential 24, the source of gridbias for tube 2! has been fixed at such a value that only the mostpositive swings of anode I!) will unbias tube and cause a signal toappear at output 21, pulse 2 does not affect output 2'! of thediscrimination circuit. Pulse 9, the desired signal, is of an amplitudesuch that the potential of grid I2 of tube 6 is driven to the inflectionpoint b of Fig. 3. The application of potential b to grid l2 causes amaximum flow of tube 6 plate current in the direction such as to givethe strongest positive signal at anode Ill and this maximum positiveswing is sufficient to unbias tube 20. Pulse g therefore causes platecurrent flow in tube 20 and a corresponding negative signal at output 2?of the discrimination circuit as indicated by pulse 2' of wave form 30.Pulse h is of an amplitude such that the potential of grid l2 of tube ,6is driven into the positive transconductance region b to c of Fig. 3such as potential It. Potential k at grid l2 will cause a net platecurrent flow such that anode I0 is positive with respect to quiescentpotential but the positive swing of anode I0 is less than that whichwould have occurred had grid l2 been driven only to potential 12. Pulseh therefore fails to unbias tube 20 and has no effect on output 21 ofthe discrimination circuit.

Thus the discrimination circuit shown in Fig. 2- acts to discriminateagainst pulses of too low an amplitude and of too great an amplitude andto discriminate in favor of pulses of a definite predetermined amplitudecharacteristic controllable as hereinbefore described. This circuit alsoacts to discriminate against pulses of negative polarity although thedetected output of high frequency stage 2 of Fig. 1 would, in theembodiment shown, consist only of positive pulses.

It will be apparent that an amplitude discrimination circuit constructedin accordance with the teachings of this invention will have a widevariety of applications in radio, radar, television, and otherelectronic fields whenever discrimination between potential variationsis desirable and the amplitude characteristics or the amplitude andpolarity characteristics of said potential variations can be used as thebasis for such discrimination. It will also be apparent that anamplitude discrimination circuit constructed in accordance with theteachings of this invention may be used in combination with othercircuits, also discriminatory in response, whose action is based onother characteristics of the input signal such as time duration or rateof change.

It will also be apparent to those well versed in the art that theparticular amplifier stage represented by tube 20 and associated circuitcomponents in Fig. 2 may be replaced by a variety of means responsiveonly to the positive excursions of anode ID of tube 6 as hereinbeforedescribed without exceeding the scope of this invention.

Since certain further changes may be made in the foregoing constructionsand different embodiments of the invention may be made without departingfrom the scope thereof, it is intended that all matters shown in theaccompanying drawings or set forth in the accompanying specificationshall be interpreted as illustrative and not in a limiting sense.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposesWithout the payment of any royalties thereon or therefor.

What is claimed is:

1. A means of pulse amplitude discrimination, comprising a vacuum tube,means providing the transconductance characteristic of said tube with aninflection point, said inflection point defined by a negativetransconductance region on one side thereof and a positivetransconductance region on the other side thereof, means biasing saidtube so that input signals of the preferred amplitude drive the tube tothe point of inflection in said transconductance characteristic, and anamplitude responsive device arranged to operate from the output of saidtube and adapted to respond only when said input signals drive said tubeto said point of inflection.

2. A means of pulse amplitude discrimination, comprising a vacuum tubehaving at least a cathode, an anode, a control electrode and one otherelectrode, a source of potential, means connecting attests said anode toapQi-nt: of positive potential. on said source, means connecting saidother electrode to a; point of higher positive potential on said source,so as to provide said tube with a point of inflection in itstransconductance eharacteristic, said point: of inflection defined, by anegative transconductanee region on one side thereof and a positivetransconductance regionon the other side thereof, means biasing saidtube so thatinput signals of the preferred. amplitude drive the tube tothe point of inflection in said transconduetance characteristic, anamplitude responsive. device, arranged to operate from the output ofsaid tube and adapted to respond only when said signals drive said tubeto said point of inflection.

3. A means of pulse amplitude discrimina Qn, comprising a vacuum tubehaving at least a cathode, an anode, a control electrode and one otherelectrode, a source of potential means connecting said anode to a pointof positive otential on said source, means connecting said otherelectrode to a point of potential at least twice as positive as saidanode, so as to provide said tube with a. point of inflection in itstransconduetance characteristic, said point of inflection defined by anegative transconductance region on one side thereof and a positivetransconductance region on. the other side thereof, means biasing saidtube so that. input signals of the preferred amplitude drive the tube tothe point of inflection in said. transconductance characteristic, anamplitude responsive device arranged to operate from the output of saidtube and adapted to respond only when said signals drive said tube tosaid point of inflection 4. A means of pulse amplitude discrimination,comprising a first vacuum tube, means. providing the transconductancecharacteristic of said, tube w h an. inflection noint: inflectiondefined bra. negative transconductance. eeion o one side thereoi and aosit ve transc nduetanc region on the other side thereon ea -s. ias nsaid tube so that. input ignal of the preferred mplitude driv said firsttub to th o nt o inflection. in said transconductance char ct ri t c,and a second vacuum tube arranged to operate from the u put of. said;first tube and adapted t respond. only when said input signals. to saidfirst tube drive the, latter to said; point of inflection. 5., A. meansoi pulse amp itude discr m na io comprising a. vacuum tube,v meansproviding; th r nsconductance charact ristic of a ube w th an inflect onpoint with a negative trans onduc ance re ion. on. one sid thereof and apos ive t anseonduct nc egion. on the other e. thereof. means forregulating the s ope of said transonductancc charac er stic, means biang s i tube so that. inputsiena s cf. the pr erred amp-1i.- tude drivethe tube to the point of inflection. in said transconductanc chaacteristic, nd an amplitude responsive device arranged to operate from.the output. or said tube and adap ed to respond only when said, inputsignals dr ve said tube to. said point 0i infle tion. V

CQNR D HQETEPNER REFERENCE-S CITED The following references; are of recod. the file of this patent:

UNITED STATES PATENTS

